The present invention relates to a self-centering device for supporting and guiding bar stocks or tubular elements, hereinafter referred to as "bar support device", while the same bar stocks or tubular elements are being fed to a turning spindle of a machine tool. In particular the invention is directed to a self-centering device able to adapt to bars or tubular elements having different cross-sectional shape and size, to constantly exert a self-centering dynamic action which maintains the bar or the tubular element perfectly aligned with the feeding path and the working axis of the spindle of the machine tool, irrespective of the diameter or cross-sectional of the same bar or tube. For the purposes of the present description, the term "bar" refers to any elongated stock element having a solid circular, square, polygonal or tubular cross-sectional configuration or outline.
In the field of machine tools, more particularly for single-spindle and multi-spindle lathes, various automatic bar feeding systems have been developed, having bar support means capable of adapting automatically, or by replacing support jaws, to guide and support bars of different diameters or cross-sectional. Examples of these devices can be found in previous patents: EP-A-213,659, EP-A-370,240, EP-A-384,344, EP-A-485,902 and EP-A-570,830.
The previously known bar feeding and supporting devices usually operate at the low working speeds of the machine tools. They nevertheless suffer limits and disadvantages in that they provide a minimal static centering action only, being not very suitable for the increasingly high working speeds of modern machine tools, which in some cases can reach or even exceed ten thousand revs per minute. In fact, at high rotation speeds, small eccentricities or slight bendings of a bar can entail strong stress and wobbling which cause noise, wear of the guides and possible machining inaccuracies.
From the patent U.S. Pat. No. 4,058,036 a variable opening and adjustable bar support device is known, comprising a tubular sleeve element and a plurality of spherical rings pivotally supported and axially spaced inside the sleeve element, between which sliding thrust rings are provided; the thrust rings have projections acting on the spherical rings to swing them in unison in a plurality of angularly oriented positions. By manually acting on a regulation ring nut, or by providing a centrifugal actuator, it is possible to vary the angular position of the spherical rings and adjust their passage opening according to the diameter of the bars to be supported and guided.
Such a bar support device, although allowing to maintain the bars substantially aligned with the working axis of the machine tool, irrespective of the diameter of the bar or tubular element, in practice suffers a number of problems caused by its structure and by the fixed arrangement of the spherical rings in relation to the rotating bar.
In fact, the variable opening bar support device according to U.S. Pat. No. 4,058,036, which forms the state of the art closest to the present invention, involves an extremely complex structure, wholly unsuitable for operating at the high working speeds of modern machine tools, both due to the considerable wear in the sliding contact between the rotating bar and the fixed guide rings, as well as to the difficulty in adjusting correctly the angular position of the various rings, causing inadequate conditions for the centering of bars. Again, the increasingly high working speeds of the modern machine tools, particularly for bars of very small diameter, will tend to increase the centrifugal forces and frictional wear between the bar and the support ring, increasing noise and wobbling problems; this is presumably due to the absence of any dampening action by the same annular element.
From tests and experiments conducted by using a bar support device having a fixed annular member for supporting rapidly rotating bars, it was concluded that randomly oriented frictional forces and the same reaction of the annular element which tends to oppose the movement of the bar, during rotation, will not improve the noise and wobble dampening action of the device.
From further tests it has been discovered that a substantial noise and wobble reduction, as well as an appropriate self-centering action, may be favourably obtained by accompanying instead of opposing the rotation of the bar by the same supporting and guiding device, as well as by providing variable contact points between the bar and the annular element according to angular disposition of the same bar supporting device.
The object of the present invention is therefore to provide a bar support device for guiding bars to the spindle of a machine tool, which make use of an angularly adjustable annular element, which exploits the rapid rotation of the bar to exert an effective and constant dynamic self-centering action on the same bar, substantially reducing problems of wear and noise caused by wobbling of the bar while it rotates at the high working speeds of the machine tool.
A further object of the present invention is to provide a self-centering device for supporting rotating bars, as referred above, which is of an extremely simple construction, and can be variously applied alone or in combination with other similar devices, to keep the bar duly centered near the spindle of the machine tool, or for guiding and supporting bars in bar-feeders for single-spindle and multi-spindle machines.
A further object of the invention is to provide a bar support self-centering device, as stated above, which is of simple and reliable construction allowing at the same time a rapid and easy set-up.